Muscle Excitation/Contraction (E/C Coupling)

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Summary
Full-Length Text

skeletal muscle excitation and contraction coupling

Summary

Key Structures of the muscle cell

See: Muscle Cell

• The sarcolemma (its plasma membrane).
• A myofibril.
  • The A band refers to the length of the thick filaments, "think "A" for d-a-rk – they are aniosotropic (or birefringent) in polarized light.
  • The I band is the region along the thin filaments (between the thick filaments). Think "I" for L-i-ght: they are "isotropic" (do not alter polarized light).
• Sarcoplasmic reticulum (SR)

• Form web-like rows.
• Store calcium.
• Are a key component to coupling muscle cell excitation to myofibril contraction.

• Terminal cisternae (aka lateral cisternae) of the sarcoplasmic reticulum.
  • Flank the transverse tubules (T-Tubules)
• Transverse tubules (T-Tubules)
  • Are tubular invaginations of sarcolemma.
• Sarcoplasm
  • The muscle cell cytoplasm.
  • Muscle cell nuclei lie within the periphery of the cell.
  • During development, the nuclei transition from a central location to a peripheral one.
  • Muscle cells comprise numerous mitochrondria: they are the energy powerhouses of the cell – the site of aerobic respiration, which by definition requires oxygen but is capable of generate the most amount of ATP.
• The synaptic terminal (aka terminal bouton, terminal button, synaptic bouton) of a typical motor nerve.
  • Within it are molecules of acetylcholine.

Key aspects of myofilaments

See: myofilaments

Thin filaments

• Actin
  • Spherical molecules joined in pairs of strands (like beads on a string). It is referred to as F-actin for filamentous actin, and comprises a polymer of G-actin monomers that are arranged in a double helix.

Tropomyosin

• Threadlike strands

Troponin

• Protein complexes that bind tropomyosin, actin, and also calcium (show their calcium-binding sites).

Thick filaments

• Comprise myosin molecules, which form a golfclub shape, and comprise two heavy chains and two light chains.

Excitation

Acetylcholine binds a post-synaptic receptor on muscle.

• This triggers an action potential, which proceeds along the T Tubule to a dihydropyridine receptor (it is blocked by dihydropyridine, hence its name).
• Depolarization of the dihydropyridine receptor activates the ryanodine receptor (aka foot proteins, calcium-release channels) within the terminal cisternae,
• This triggers the release of calcium into the cytosol.

Calcium binds troponin.

• This causes a shift in tropomyosin, moving it away from its blocking position along actin, which allows myosin to bind actin.

Myosin binds actin and proceeds through thin filament sliding (muscle contraction).

Drawing at the end.

Tropomyosin shift

• Once the action potentials cease, calcium releases from troponin and returns to the cytoplasm, tropomyosin then shifts, again, and once again myosin is unable to bind actin, and the contractions cease.

Huxley Sliding-Filament Model

See: Huxley Sliding-Filament Model.

The rigor state.

• The myosin head is bound to the thin filament.
• Calcium is bound to troponin.
  • Calcium binding to troponin allows myosin access to its binding site on actin.

ATP induces release of actin.

• Myosin has ATP bound to its head.
• The actin molecules are separated from (no longer bound to) the myosin.
• ATP is required to move out of the rigor state.
• If ATP is absent, which occurs after death, rigor will persist, called rigor mortis.

ATP is hydrolyzed to ADP and Inorganic phosphate (Pi).

• The myosin head rotates on the neck: it is now "cocked" – it's in its high-energy state.
• The "cocked" state causes the thin and thick filaments to again bind via their cross-bridge.
• ADP and inorganic phosphate (Pi) are still bound to the myosin head.

Pi release initiates the power stroke for the myosin head to release its energy.

• Accordingly, the thin filament begins its slide.

The myosin returns to its uncocked, low energy state.

• At some point after the power stroke, ADP is released.
• Note that this is an area of intertextual variation, some authors instead write that ADP is released at the same time as phosphate to initiated the power stroke.

Full-Length Text

• Here, we'll learn the coupling of skeletal muscle excitation and contraction.

• First, draw a longitudinal view of a muscle cell in three dimensions, so we can understand the environment of the cell.

• Label its sarcolemma (its plasma membrane).

• Next, draw a complex form of a myofibril.

First, show its external features:

• Designate the A band, which refers to the length of the thick filaments, "think "A" for d-a-rk – they are aniosotropic (or birefringent) in polarized light.

• Then, designate the I band, which is the region along the thin filaments, between the thick filaments.
  • Think "I" for L-i-ght – they are "isotropic" (do not alter polarized light).

• Now, draw web-like, longitudinal rows of sarcoplasmic reticulum (SR), which stores calcium and is a key component to coupling muscle cell excitation to myofibril contraction.

• Then draw bands of transversely-oriented terminal cisternae (aka lateral sacs) of the sarcoplasmic reticulum.

• And show that they flank transverse tubules (T-Tubules); they are tubular invaginations of sarcolemma.

• Now, show that cytosol constitutes the internal label the internal milieu of the myofibril.

• As a corollary, show that sarcoplasm fills the internal milieu of the muscle cell.

• Specifically, include a couple of mitochondria within the muscle cell: they are the energy powerhouses of the cell – the site of aerobic respiration, which by definition requires oxygen but is capable of generate the most amount of ATP.

• Next, draw the synaptic terminal (aka terminal bouton, terminal button, synaptic bouton) of a typical motor nerve.

• Within it, draw molecules of acetylcholine.

Now, let's draw some key aspects of myofilaments, which are crucial to our understanding of excitation and contraction.

• Indicate that thin filaments notably comprise:
  • Actin, which are spherical molecules joined in pairs of double helical strands (like beads on a string),
  • Tropomyosin, which are threadlike strands, and
  • Troponin protein complexes that bind tropomyosin, actin, and also calcium (show their calcium-binding sites).

• Next, indicate the myosin binding sites on actin.

• There is also an ATPase site, an ATP-splitting site.

• Next, show that thick filaments of myosin, which constitute heavy and light chains form a golfclub shape, as such indicate their:
  • Head, neck, and tail.

• Before we proceed with excitation, show that the thin and thick filaments are unbound.

Now, show the steps of excitation:

• Show that acetylcholine binds a post-synaptic receptor on muscle.
  • This triggers an action potential, which proceeds along the T Tubule to a dihydropyridine receptor (it is blocked by dihydropyridine, hence its name).
  • Depolarization of the dihydropyridine receptor activates the ryanodine receptor (aka foot proteins, calcium-release channels) within the terminal cisternae, which triggers the release of calcium into the cytosol.

• Show that calcium binds troponin.
  • This causes a shift in tropomyosin, moving it away from its blocking position along actin, which allows myosin to bind actin.

• Myosin binds actin and proceeds through thin filament sliding (muscle contraction), which we draw at the end.

• Show that, then, once the action potentials cease, calcium releases from troponin and returns to the cytoplasm, tropomyosin then shifts, again, and once again myosin is unable to bind actin, and the contractions cease.

Now, let's draw key steps in the Huxley Sliding-Filament Model.

• Draw a myosin head bound to the thin filament.

• Next, we see that ATP induces release of actin.

• Redraw myosin with ATP bound to its head.

• Then, draw the actin molecules as separated from (no longer bound to) the myosin.

• Thus, we see that ATP is required to move out of the rigor state.

• Write that if ATP is absent, which occurs after death, rigor will persist, called rigor mortis.

• Next, show that ATP is hydrolyzed to ADP and Inorganic phosphate (Pi) prompts the myosin head to rotate on the neck: it is now "cocked" – it's in its high-energy state.

• Now, show that the "cocked" state causes the thin and thick filaments to again bind via their cross-bridge.

• Show that ADP and inorganic phosphate (Pi) are still bound to the myosin head.

• Redraw this set-up but show that Pi release initiates the power stroke for the myosin head to release its energy.

• Indicate that accordingly the thin filament begins its slide.

• Finally, redraw the myosin in its uncocked, low energy state.

• At some point after the power stroke, ADP is released.
  • Note that this is an area of intertextual variation, some authors instead write that ADP is released at the same time as phosphate to initiated the power stroke.